This invention relates to the measurement of flow rate of a fluid and more particularly to measuring flow rate using unsteady pressure measurements.
In many industries it is desirable to measure the flow rate of a multiphase fluid. In the oil and gas industry, or comparable industries, for example, it is desirable to measure the flow rate of multiphase fluids, especially fluids having three phases, such as oil, water, and gas. It is known also to measure the flow rate of certain fluids (one or more liquids and/or gases) in a pipe using cross-correlation flow meters. Such meters measure an element of the flow that moves or convects with (or is related to) the fluid flow (or a group of fluid particles). The meter measures this element at two locations separated by a known distance along the flow path and then calculates the time for such element to move between the two locations. The time delay is determined by a cross-correlation of the two measured signals. A velocity is then determined by the distance between the measurements divided by the time delay. The flow velocity is then related to the flow rate by calibration.
One such cross-correlation meter that measures flow rate in a multiphase flow is described in U.S. Pat. No. 5,591,922, entitled xe2x80x9cMethod and Apparatus for Measuring Multiphase Flow,xe2x80x9d to Segeral et al., issued Jan. 7, 1997. In that case, a pair of venturis are located a predetermined distance apart which induce a change in flow speed through the venturi and a resulting pressure difference (or delta-P) across each venturi, which are measured. The delta-P pressure signals measured at each venturi are cross-correlated to determine the time delay which is indicative of the total volume flow rate. However, such a technique requires a change in the flow properties (e.g., flow velocity or density) at the two measurement points to make the measurement. Also, the delta-P is generated with an area contraction or constriction, and is not a naturally occurring observable characteristic of the fluid.
Objects of the present invention include provision of a system for measuring the flow rate (or velocity) of fluid flow in pipes.
According to the present invention, an apparatus for measuring a velocity of a fluid moving in a pipe comprises a first filter which measures a vortical pressure field at a first axial location along the pipe which provides a first pressure signal indicative of the vortical pressure field; a second filter which measures the vortical pressure field at a second axial location along the pipe which provides a second pressure signal indicative of the vortical pressure field; and a signal processor, responsive to the first and the second pressure signals, which provides a velocity signal indicative of a velocity of the vortical pressure field moving in the pipe.
According further to the present invention, the first and the second filters pass wavelengths associated with the vortical pressure field and not associated with an acoustic pressure field. According further to the present invention, the first filter comprises a first spatial filter; and the second filter comprises a second spatial filter. According still further to the present invention, the vortical pressure field comprises a vortical pressure field. Still further according to the present invention, the first and the second filters pass wavelengths associated with the vortical pressure field and not associated with an acoustic pressure field. According further to the present invention, the spatial filter filters out wavelengths above a predetermined wavelength. Still further according to the present invention, at least one of the pressure sensors comprises a strain gauge disposed on a surface of the pipe. Further according to the present invention, the strain gauge comprises a fiber optic strain gauge.
Still further according to the present invention, the signal processor comprises logic which calculates a cross-correlation between the first and the second vortical pressure signals and provides a time delay signal indicative of the time it takes for the vortical pressure field to move from the first location to the second location. Further according to the present invention, the velocity signal is indicative of the velocity of the fluid moving in the pipe.
The present invention provides a significant improvement over the prior art by providing a measurement of the average flow rate of fluid flow in a pipe or other conduit (where a fluid is defined as one or more liquids and/or gases) without requiring a flow restriction in the pipe or any other change in the flow velocity of the fluid.
The present invention determines a convection velocity by measuring unsteady (or dynamic or ac) pressures and extracting the pressure signal indicative of a vortical pressure (or flow) field (or perturbation) which convects at or near the average velocity of the fluid. The vortical pressure field is then used to determine the convection velocity by cross-correlation techniques, such convection velocity being proportional (or approximately equal to) the flow rate of the fluid. If needed, the flow rate of the fluid may then be determined by calibrating the convection velocity to the flow rate.
The invention may be used to measure the velocity of any inhomogeneous flow field, such as gas bubbles, gas slugs, particles, or chunks of material, and its associated pressure field that propagates in a flow provided the spatial filters have a separation within the acceptable coherence length of the flow field to be measured and the sensor spacing within each spatial filter is longer than a characteristic spatial (axial or transverse) length of the flow field. Also, the invention may be used to detect different flow rates within the same mixture (e.g., the flow rate of a vortical pressure field as well as other inhomogeneous pressure fields).
Also, the invention may be used with any combinations of liquids and/or gases. The invention may be used in a flow meter for use in oil or gas wells to determine the flow rate of a multiphase fluid, such as a three phase fluid of oil, water, and gas. The invention will also work in any other environment or applications or any other fluids (one or more liquids and/or gases) or mixtures. The invention will work with any pipe or tube or with any conduit that carries fluid. Also, the invention has no inherent flow range limitations, and, as such, can measure very low flow rates and has no maximum flow rate limit. The invention will also work if the fluid is flowing in either direction in the pipe. Further, the invention may be used directly on a pipe or on a tube inserted into a flowing fluid.
The foregoing and other objects, features, and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.